Old English fullice "entirely; perfectly; completely;" see full (adj.) + -ly (2). Of similar formation are Dutch vollijk, German ööllig, Danish fuldelig.
he didn't fully understand
fully grown
the evidence amply (or fully) confirms our suspicions
they were fully (or amply) fed
full-grown
fulling
full-length
fullness
full-time
fully
fulminant
fulminate
fulmination
fulness
fulsome
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